In networked computer systems, services are commonly provided by service providers to service consumers or requestors. The interaction between service providers and requestors is governed by a protocol, such as the Web Service protocol defined by the World Wide Web Consortium (W3C). Such protocols provide for policies that detail the allowed configurations of a service, that is, the technical requirements of interactions between the service provider and service requestors. Policies may define features or elements such as the required security, quality of service, or transport protocol that are either required or preferred in a service interaction. Thus, the requester of a service can obtain the relevant provider's policy and specify their service requests in accordance with the policy.
However, one problem is that not all combinations of alternative policy features or elements may be valid. In addition, to determine the alternative features selected by a requester may require analysis of the request messages, which uses significant processing power. Furthermore such message analysis may not always provide a definitive indication of the alternative policy elements being used by the requester.